Ceramic coated metal bodies



Oct. 30, 1962 N. J. GRANT CERAMIC COATED METAL BODIES Filed Sept. 16, 1959 a mm? -M-- F M 0% J mr /w g M11! t, l 5% 054 a s n w? a t 7, Z r a A WW 0 M f M g, 9 WNW INVENTOR. lV/CIVUAJS J WW7- {WW United States 3,061,482 CERAMIC COATED METAL BODIES Nicholas J. Grant, Leslie Road, Winchester, Mass. Filed Sept. 16, 1959, Ser. No. 840,370 9 Claims. (Cl. 1486) This invention relates to metal-base surfaces having an adherent layer of metal oxide and, in particular, to metal structures having a strongly adhering ceramic coating or enamel, frit or glaze of thickness sufficient to insulate said base metal againstoxidation or other corrosive atmospheres and against erosion at elevated temperatures. In addition, the invention provides heat-reflective surfaces.

Advanced power plants for aircraft and missiles require protection of metal components in the combustion area of the engines at temperatures up to 4000" F. or higher. Gas temperatures over 2000" F. make it necessary to protect the base metal by using certain protective coatings, such as ceramic, of thicknesses sufficient to insulate the base metal structure from severe oxidation and strength reduction defects.

Coatings have been developed by flame-spraying ceramic oxide onto a metal surface. Others have been proposed utilizing a metal-reinforced ceramic applied by trowelling a ramming mix into a metallic matrix (e.g. stainless steel mesh or screening) attached to the surface of a base metal to be protected. Still others have been proposed wherein fine metal powders are incorporated into ceramic or glassy coatings for additional adherence to the base metal.

The durability and performance of any protective coating willl vary with duration and temperature of exposure, with variation in gas composition and velocity and with the content of abrasive or erosive particles in the gas. For example, a coating for turbine blades and guide vanes made of molybdenum should have the following requisities:

It should be dense and pinhole free to confer oxidation resistance to the underlying molybdenum surface. The coating must of high melting point and should be capable of withstanding rapid changes in temperature without spalling. Thus, the difference in coefficient of thermal expansion between the base metal and the coating is an important consideration. In addition, the coating must be capable of absorbing the impact of small, high velocity particles without appreciably eroding away and exposing the underlying metal.

Bonding is another important consideration. Generally, this is achieved by mechanically preparing the metal surface in order to promote mechanical bonding between the metal oxide coating and the base metal, this being necessary in the case of refractory oxides such as A1 0 ZrO CeO ZrSiO, which are substantially chemically neutral with respect to the underlying base metal. However, mechanical bonding to the base metal itself presents spalling problems where the coefficients of expansion between the coating and the metal differ appreciably.

'I have now discovered a method whereby I can produce an improved oxide coated metal base structure in which the coating is strongly bonded to the base metal surface.

It is the object of this invention to provide a metal oxideor ceramic-coated metal product characterized by improved bonding and improved resistance to spalling under environmental conditions involving heat shock.

Another objects is to provide a method for bonding a metal oxide coating whether glassy or crystalline or mixtures thereof to a metal base surface by preparing the metal surface to effect an improved bond.

These and other objects will more clearly appear from the following disclosure and the appended drawings, wherein:

FIGS. 1 to 3 represent the microstructure in enlarged cross section showing various means by which bonding may be obtained between a metal-base structure and metal oxide coating such as refractory oxides;

Patented Oct. 30,1962

FIG. 4 is a curve showing a relation between the preferred maximum of solute metal in the matrix metal and the corresponding free energy heat of formation of the solute oxide for internal oxidation purposes.

Broadly speaking, one embodiment of my method for producing an adherent oxide coating on a metal surface comprises providing a metal base containing at least adjacent to the surface thereof a dispersion of finely divided stable metal oxide to which surface is applied a metal oxide coating (e.g. a ceramic, frit or glaze) which is fired so that a bond is obtained between it and the dispersed oxide phase in and below the metal surface. As stated hereinbefore, the oxide coating may be an enamel, frit or glaze or may comprise as high temperature ceramics such refractory oxides as A1 0 ZrO 3Al O 2SiO (mullite), Mg SiO (forsterite), ZrSiO (zircon), CeO or other types of refractory materials. High temperature poreclain enamels consisting of a mixture of oxides of silicon, aluminum, boron, calcium, sodium, lithium and potassium may also be used.

The oxide coating may be applied as a liquid slurry by dip coating or spraying and dried and thereafter fired, or may flame-sprayed, or applied by means of a plasma jet torch, or other known means. The firing to effect bonding between the dispersed oxide phase in the metal and that in the coating may be carried out during the coating process where such coating is conducted at elevated temperatures (e.g. by the plasma jet) or it may be carried out after the base metal has been coated as in the case where the coating is applied as a slurry. In any event, the firing should be effective to sinter the oxide layer together and to bond said layer to the metal surface via the intrusion of the oxide layer into said metal base surface in connecting or bonding relationship with the dispersed oxide particles adjacent and below the metal surface. In other words, the oxide layer is anchored to the the surface through bonding with the dispersed phase.

The dispersed oxide phase in the base metal may be obtained in several ways. One method is to alloy the base metal with an amount of a solute metal which has a high propensity to combine with oxygen and then internally oxidize the base metal to convert the solute metal to a dispersed oxide phase For example, such metal might be a plate of copper containing 1% silicon which is subjected to internal oxidation to convert Si to SiO by oxidizing the copper surface to cupric oxide in air and then heating in an inert atmosphere (e.g. argon) to permit the cupric oxide to dissolve into the copper at a temperature of about 1110 F. to 1830 F. The oxygen supplied by the copper oxide surface oxidizes the Si to SiO A thickness of oxide suflicient to convert the desired amount of silicon to Si0 can be calculated by checking the weight increase of the specimen in terms of oxygen. The oxide should be kept thin and adherent.

In the case of nickel, an alternate technique is used. A mixture of nickel and nickel oxide is utilized to produce a low pressure of oxygen and internal oxidation is carried out (to convert the solute Al to A1 0 or the solute Si to Si0 or the solute Cr to Cr O etc.) at temperatures of 1470 F. to 2370 F. The transfer of oxygen is different for the two base metals copper and nickel and, therefore, requires these different techniques. Surface oxide coatings obtained by oxidizing a surface are by themselves not desirable as bonding media in view of their spalling characteristics.

The dispersed oxide produced in copper, being surrounded by a matrix of copper, serves as bonding anchors to which a ceramic coating may be made to adhere. In this case, such coating may comprise A1 which may be caused through suitable heating below the melting point of the base metal to effect a chemical bond with the SiO in the matrix, the bonding being based to some degree on a silicate compound. Or where the dispersed oxide phase and the ceramic layer are to some degree mutually soluble, then the bonding may be effected by diffusion, one into the other. Or a low melting point oxide can be used to affect the bonding of more refractory oxides, serving as a fiux or to produce a glassy bond.

The internal oxidation of the alloyed base metal may be produced just at the surface of the metal base or throughout the cross section depending on the thickness of the base metal. The size of the dispersed oxide in the matrix will depend generally on the temperature of oxidation, the coarser particles being obtained at oxidation temperatures approaching the melting point.

Instead of producing the dispersed oxide phase by in- Iernal oxidation, it may be produced by powder metalurgy.

One method comprises mixing electrolytic copper powder of minus 200 mesh size with, for example, by volume of 0.05 micron silica powder, consolidating said mixture into a desired shape by pressing or pressing followed by sintering and then hot working the shape to a plate or sheet of given dimensions. The resulting article will have dispersed substantially uniformly throughout the cross section thereof, as well as at and below the surface, particles of silica. In this instance the silica serves a two-fold function: (1) it provides a wrought, dispersion-hardened copper composition having improved resistance to creep, improved yield strength, and improved high temperature stability up to below its melting point in combination with substantially high electrical and heat conductivity; and (2) it also provides metal oxide bonding anchors by which a refractory coating, such as A1 0 is caused to adhere strongly to the copper surface. Thus, where the excellent heat sink property of copper is desired in an eroding environment maintained at an elevated temperature below the melting point of copper, the refractory-coated, dispersion-hardened copper provided by the invention is utilized. By using a heat-insulating coating of A1 0 on the silica-hardened copper, it is possible to extend the use of the wrought copper up to temperatures just below the melting point of the copper.

As stated hereinbefore, the difference in expansion coefficient between the base metal and the ceramic coating is important where optimum resistance to spalling is essential. As will be appreciated, the lower the difference the greater will be the tendency for the coating to resist spalling. Where the difference is undesirably large, I propose to overcome this difficulty by providing a high concentration of disperse oxide phase adjacent the surface of the metal. This can be achieved by internal oxidation by providing a higher amount of solute metal (a refractory oxide forming metal such as Al, Si, Cr, etc.) in the base metal and by so controlling the internal oxidation as to obtain a high concentration of disperse oxide phase adjacent the surface wherein the coefficient of expansion of the resulting composite near the surface is close to that of the oxide coating applied to it. Or, if desired, the metal base surface, for example copper, could be first prepared by aluminizing or siliconizing by deposition from a halide vapor containing one of the said metals and the metal thereafter diffused inward into the outer surface by heat treatment followed by controlled internal oxidation to produce a high concentration of disperse oxide phase at or adjacent the outer surface. By this method, it would be possible to obtain a dispersed oxide phase near the metal surface amounting up to by volume of the composition near the surface thus making available a large amount of bonding anchors. After the disperse phase has been obtained, the ceramic layer would then be applied as described above.

Where a greater bonding density per unit area is desired, I propose to achieve this by still a further method. As a first step, I would provide a metal base having a disperse oxide phase at least adjacent the surface thereof. I would then apply an intermediate coating of a mixture of oxide powder and metal powder (e.g. 5 to 25% by volume of metal powder and the balance metal oxide) to the surface and fire it in place, the metal powder of the mixture forming a bond with the metal surface and the oxide powder forming a bond with the dispersed oxide phase. Over this intermediate layer, I would add a coating of refractory oxide and fire it to form a bond with the oxide of the intermediate layer. Plural coatings may be employed to produce a graded structure in which the cross section of the coating gradually increases in metal oxide content towards the surface.

Assuming the material to be coated is iron, I would provide a sheet of silicon steel comprising 3.0% silicon. I would then subject the steel to internal oxidation by heating it to an elevated temperature, for example about 1700 F. in a partial vacuum containing oxygen at a pressure of about 10 microns to form a dispersed phase of SiO I would then coat the surface with an intermediate layer of a slurry comprising a mixture of finely divided fire clay type of oxide and carbonyl iron powder (e.g. 20% by volume of 10; iron and by volume of finely divided fire clay) and fire the intermediate coating to form a diffusion bond with the surface of the iron. Here a two-fold bonding effect is achieved, one between the metal in the coating and the metal in the base metal surface and the other between the oxide of the coating and the dispersed oxide in the base metal. With the intermediate layer enriched in Al O strongly bonded to the iron surface, I now apply a layer of SiO;, to the top of the intermediate layer and fire it to achieve a bond between it and the intermediate layer and produce a glazed surface by reaction of the silica with the fire clay. Thus, I am able to obtain a concentration gradient of the oxide ranging from a low concentration of dispersed oxide' phase in the metal matrix to a still higher concentration in the intermediate layer to a very high concentration in the outer surface. Such a controlled concentration gradient is one way of compensating for the difference in thermal expansivity between the metal and the oxide per se.

While reference has been made to protective oxide coatings for copper and iron materials, it will be appreciated that the invention need not be limited thereto as will be apparent from the following illustrative examples:

Example 1 In protecting molybdenum the following method may be employed:

Molybdenum powder of about 1 average particle size is mixed with about 10% by volume of A1 0 (about 0.05 1. average size). A given weight of the mixture is then cold pressed in a die into a coherent compact at a pressure of 40 tons per square inch, encased in a container of iron and sealed vacuum tight. The sealed compact is then heated rapidly to an extrusion temperature of about 2600 F. and extruded through a reduction die at a pressure of about tons per square inch at an extrusion ratio of 15:1.

An alternative method is to heat the iron encased molybdenum rapidly to 2600 F. (at this temperature the iron is extremely soft) followed by dropping it into a stainless steel can or sheath preheated to 2000 F. or even to 1800 F. and quickly extruding the assembly. The, outer stainless steel can being stiffer assures a good extrusion.

In any event, the piece of the extruded shape is then hot forged into the shape of a turbine blade. The surface of the forged blade is cleaned of the protective metal coating to expose the underlying dispersion hardened molybdenum and then coated with a slurry of ziron comprising 0.05 micron powder dispersed through a vehicle of absolute alcohol, the ratio of the powder to the vehicle comprising 40 parts of ziron to 100 parts of vehicle. A small amount of SiO is contained in the slurry to aid in forming a glassy bonding phase in the final coating. The

refractory oxide coating is dried and then fired at a temperature of about 2600 F. in vacuum for about 2 hours to effect bonding between the dispersed oxide phase of A1 in and near the molybdenum surface and the refractory coating as shown in FIG. 1 which represents generally the prevailing boundary conditions on an enlarged scale. The figure shows in cross section the molybdenum base portion 1 having dispersed therethrough finely divided A1 0 and the protective layer 4 of zircon bonded to the molybdenum body along the interface 3 by virtue of the bond with the A1 0 phase at and near the molybdenum surface. The use of additional silica in the zircon coating aids in the formation of a silicate bond with the alumina dispersed near the molybdenum surface.

Example 2 In preparing a structural component of titanium for use at 1200 to 1500 F. under oxidizing conditions in jet engines or in ram jets, an alloy of titanium is produced containing about 0.7% cerium. The alloy is fabricated in the usual manner into a sheet metal product of about 0.02 inch thick and the component produced therefrom. In accordance with the disclosure of my copending application Ser. No. 770,392, filed Oct. 29, 1958, the result ing shape is heated to a temperature at which the alpha solid solution of TeCe prevails, that is at a temperature in the neighborhood of 1470" F. for about 2 hours in a vacuum of about 0.25 micron of mercury, and then quickly air quenched to retain the alpha solid solution. The thus-treated shape is suitably supported and then heated to 1560 F. in a vacuum furnace at a leak rate of air to maintain a vacuum of about 10 microns for about 6 hours to internally oxidize the cerium in the titanium at least adjacent the surface to be protected to form a dispersion of CeO After this a finely divided oxide frit is prepared comprising by weight 38% SiO 44% BaO, 6.5% B 0 4% CaO, 2.5% BeO and 5% ZnO. From this frit i produced a proprietary coating referred to as NBS (National Bureau of Standards) A4017. To the frit is added 30% by weight of chrornic oxide, 5% by weight enamelers clay and the balance 48% water. The titanium surface is coated with the mix and, after drying, the coated article is heated in a vacuum furnace at a tem perature of about 2000 F. for 6 hours. As illustrative of the boundary conditions which will prevail between the refractory coating and the titanium metal base, reference is made to FIG. 2 which depicts the structure in enlarged cross section. FIG. 2 shows the titanium metal portion 5 internally oxidized near its upper surface to form a dispersion of the Ce0 phase and the layer 8 of glazed ceramic bonded to the titanium surface along the inter face 7 by virtue of the bonding action between the glazed layer and the dispersed oxide phase of Ce0 adjacent the titanium surface.

Example 3 In producing a ceramic coating of A1 0 on nickel, the following procedure may be employed.

A nickel surface is siliconized with a flash coating of silicon by heating the nickel surface in a resistance wound tube furnace at a temperature of about 2000 F. for about 30 minutes in the presence of an atmosphere containing SiClr in a train of H gas saturated at a temperature of 100 F. The coated nickel surface is then subjected to a diffusion heat treatment in a hydrogen atmosphere at a temperature of about 2000 F. for about 6 hours and then internally oxidized by heating in a vacuum in a NiNiO mixture to give oxygen pressure at 1460" F.

6 to 2370 F. sufficient to effect diffusion thereof into the nickel surface. The heating is conducted for about hours at 1460 F. and about 8 hours at 2370 F. The surface to be protected is cleaned and then coated with a slurry of A1 0 plus binder (about 0.1 micron in size), the slurry comprising 40 parts of A1 0 to 100 parts of vehicle comprising absolute alcohol. After the coating is dried, the coated nickel is subjected to firing in an inert atmosphere at a temperature of about 2200 F. for about 4 hours to effect bonding between the A1 0 coating and the dispersed SiO phase in the nickel. This method has its advantages in that a relatively high concentration of SiO phase can be produced near the nickel surface by first diffusing silicon into the nickel surface so that a desired concentration of solute metal is obtained near the surface for subsequent conversion into dispersed oxide phase. The somewhat high concentration of SiO phase near the surface is depicted in FIG. 3 which shows the nickel portion 9 with a high concentration of the SiO phase 10 near the surface and the ceramic A1 0 coating 12 bonded to the nickel surface along the interface 11 by virtue of the bond with the highly concentrated dispersed phase.

As stated hereinbefore, many variations may be resorted to in applying the oxide coating. For example, in Example 3, instead of applying the Al O coating as a slurry followed by firing, it may be applied by a plasma jet flame at temperatures in the range of 4000 to 6000 F. this being the temperature of the particles of A1 0 impinging upon the nickel surface, the residual heat in the applied coating being utilized to obtain the necessary bonding by diffusion with the Si0 particles in the nickel matrix.

The dispersed hard phase in the base metal is preferably derivedfrom metals having a high propensity to form a stable refractory oxide of high melting point. Thus, where the disperse hard phase is obtained by internally oxidizing a base metal having alloyed therewith a refractory oxide-forming metal (e.g. Al, Si, Zr, etc.) the negative free energy of formation of the refractory oxideforming metal with oxygen at 25 C. should be at least about 90,000 calories per gram atom of oxygen and generally at least about 120,000 calories. Thus, the free energy of formation of SiO is 96,200 calories, of TiO -101,400 calories, of ZrO 122,200 calories, of A1 0 (alpha) -125,590 calories, of MgO -136,000 calories, of BeO 139,000 calories, and Th0; in the neighborhood of 145,000 calories.

Generally, the matrix metal will have a negative free energy of formation with oxygen not exceeding 70,000 calories per gram atom of oxygen. It is this wide difference between the free energy of formation of the matrix metal oxide and the refractory oxide that makes it possible to oxidize a refractory oxide-forming metal alloyed with the matrix metal in preference to the matrix metal.

However, in some situations it is possible to preferentially oxidize a refractory oxide-forming metal where the base metal also has a high free energy of formation of the oxide. Such a metal is titanium. By alloying up to about 1% Ce with titanium (preferably 0.5 to 1%) and subjecting the alloy to oxidation in a vacuum of known leak rate of air (for example a leak rate corresponding to 10* mm. Hg of pressure, it is possible to preferentially oxidize the Ce to CeO This is because oxygen has a high solubility in titanium (up to about 15% and considerable amounts of oxygen must be dissolved before any of the titanium is converted into TiO By having Ce present, and by controlling the partial pressure of oxygen, CeO is preferentially formed. What has been said regarding titanium is also applicable to zirconium.

The amountof disperse refractory oxide employed in the matrix metal may range up to about 15% or 20% by volume, generally from about 3% to 15 by volume. I prefer to use an amount ranging from about 8% to 15 by volume. The foregoing amounts are easily obtainable by powder metallurgy by mixing the desired amount of oxide with a given amount of matrix metal powder, consolidating the mixture into a compact and working the compact to the desired shape.

Where the dispersed oxide is produced by internally oxidizing a matrix metal containing an amount of oxidizable solute metal, generally the maximum amount of solute metal which would be supplied in the matrix metal will depend upon the negative free energy of formation of the solute metal oxide. As stated above, the negative free energy of formation of the matrix metal oxide per gram atom of oxygen should not exceed about 70,000 calories, while that for the solute metal oxide should at least be about 90,000 calories. I have found, for internal oxidation purposes, that larger amounts of solute metal can be used in the matrix metal where the oxide of the solute metal has a negative free energy of formation of less than 110,000 calories.

For example, if nickel is the matrix metal and chromium is the solute metal, the preferred maximum of chromium alloyed with nickel would be about 9% by weight, chromium oxidized to Cr O having a negative free energy of formation in the neighborhood of about 90,000 calories at 25 C. I prefer to use about 4% chromium.

Similarly, where silicon is used as the solute metal with a matrix metal such as nickel, its preferred maximum would be about 4 to 5% by weight, silicon oxidized to SiO having a negative free energy of formation of about 96,200 calories. I prefer to use about 2% silicon.

Where aluminum is used as the solute metal, because its oxide (A1 has a negative free energy of formation of about 125,590 calories, the preferred maximum should not exceed about 3% by weight of Al in the matrix metal. I prefer to use about 1.5% Al.

With respect to the foregoing, reference is made to FIG. 4 which depicts the relation between the solute metal in the matrix metal and the corresponding negative free energy of formation of the solute oxide. As illustrated by FIG. 4, the amount of solute metal in the matrix metal prior to internal oxidation should preferably not exceed the limits indicated by curve A. Generally, the amount of solute metal employed will be at least 0.5% and preferably range from about 1% by weight to the maximum defined by curve A when related to the negative free energy of formation of the oxide of a given solute metal.

The protective oxide coating produced in accordance with my invention may range in thickness up to about one-sixteenth of an inch and generally from about 0.005 to 0.06 inch. The invention is preferably applicable to to the production of coatings ranging in thickness from about 0.01 to 0.03 inch. The firing temperature employed for the coating may range from about 70% to 95% of the melting point of the coated metal.

Where thicknesses on the high side are required, these may be accomplished by plural coatings. Where an intermediate coating is prepared using a metal powder mixed with the oxide, the amount of metal powder may range from about to 25% by volume of the mixture. Particle sizes of the order of about -325 mesh may be employed for the metal powder, although the metal powder need not be limited to that size.

The matrix metal or alloy to which the invention is applicable should have a melting point of at least about 1000 C. Such metals may include copper, nickel, iron, cobalt, molybdenum, niobium, tantalum, tungsten and chromium. Alloys based on these metals may likewise be treated. The invention is also applicable to such metals as titanium and zirconium, particularly where the disperse oxide is produced by the internal oxidation of such solute metals as thorium, cerium, lanthanum and similar rare earth metals alloyed therewith.

As pointed out hereinbefore, several methods may be employed for internally oxidizing the matrix metal. For an alloy of Ti-Ce, this can be achieved in an atmosphere of controlled oxygen partial pressure, such as would be achieved by a leak rate of 10 mm. Hg of air in a vacuum furnace. Or where the alloy is Cu-Si, the oxygen may be caused to diffuse inward to oxidize the Si by first oxidizing the alloy in air to produce Cu O followed by heating the oxidized alloy in an inert atmosphere to decompose the Cu O to diffuse oxygen into the copper matrix for reaction with the silicon. Or where the alloy is NiSi, the internal oxidation may be achieved by heating the alloy in the presence of a mixture of NiNiO, the oxygen pressure being controlled by the ratio of Ni to NiO. Thus, it is apparent that various methods may be employed to suit the particular matrix metal used.

While it has been disclosed that the disperse oxide at or near the surface of the base metal helps in achieving bonding of an oxide coating applied to the base metal surface, it will be appreciated that the disperse oxide will also aid to bond the natural oxide of the base metal. For example, assuming a nickel surface has been prepared containing a disperse oxide of SiO this disperse oxide would also help in forming an adherent coating of NiO on the surface. After the disperse oxide has been provided, I would then oxidize the nickel surface by heating it in air to form NiO which would be strongly bonded to the surface via the SiO anchors in and below the surface. An adherent coating of copper oxide could similarly be formed on copper and similarly with other metals.

The present invention is applicable to the enamelling art. Porcelain enamels have long been popular as engineering materials because of their combination of protective and decorative properties. In order to insure adequate bonding of enamels on certain irons, the irons are processed in special rolls that impart a tooth-like finish to the surface to be enameled. With my invention, I rely on the dispersed metal oxide in the metal to achieve the bonding with the enamel.

My invention is particularly useful in the production of heat resistant, ceramic-coated metals and alloys for use in the following applications among others:

(1) Afterburner combustor components and combustion chambers for ramjet engines;

(2) Burner tubes;

(3) Pocket thermostat chambers and exit nozzles;

(4) Thermocouple tubes;

(5) Interior and exterior surfaces of note cones;

(6) Fuel ejectors;

(7) Turbine blades; 8) Fire walls.

When speaking of metal oxide coatings herein it is meant to include any inorganic oxide coating, be it a porcelain enamel, a metal silicate, a simple metal oxide, aluminates, and the like.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

What is claimed is:

1. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a metal base containing at least adjacent the surface thereof a dispersion of finely divided stable metal oxide having a negative free energy of formation at about 25 C. of at least 90,000 calories per gram atom of oxygen, producing an oxide coating on said metal surface, and thermally bonding said coating to said metal surface via bonds formed between said oxide coating and the dispersed metal oxide at least adjacent the metal base surface.

2. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a metal base having alloyed therewith a refractory oxide-forming solute metal in an amount by weight ranging up to the maximum determined by curve A of FIG. 4, said solute metal being capable of forming by oxidation a finely ivided disperse stable metal oxide and having a negative free energy of formation at about 25 C. of at least 90,000 calories per gram atom of oxygen, subjecting said metal base to internal oxidation at least adjacent the surface thereof to form a finely divided dispersed oxide phase of said solute metal, coating the metal base surface with a layer of metal oxide and thermally bonding said metal oxide to said surface by forming a bond between said layer and the finely divided dispersed oxide phase at least adjacent the metal base surface.

3. The method of claim 2, wherein the amount of refractory oxide-forming metal in said base metal is at least 0.5% by Weight of the composition of the metal base.

4. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a batch of metal powder, mixing with said powder up to about 20% by volume of a finely divided stable metal oxide having a negative free energy of formation at about 25 C. of at least about 90,000 calories per gram atom of oxygen, consolidating and working said powder mixture to produce a wrought shape having dispersed therethrough and adjacent the surface thereof said finely divided stable metal oxide, coating the metal surface with a layer of metal oxide and thermally bonding said metal oxide to said surface by forming a bond between said layer and the dispersed metal oxide adjacent said wrought metal surface.

5. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a batch of metal powder, mixing with said powder about 3% to 15% by volume of a finely divided stable metal oxide having a negative free energy of formation of at least about 90,000 calories per gram atom of oxygen, consolidating and working said powder mixture to produce a wrought shape having dispersed therethrough and adjacent the surface thereof said stable metal oxide, coating the metal surface with a layer of metal oxide and thermally bonding said metal oxide to said surface by forming a bond between said layer and the finely divided dispersed metal oxide at least adjacent said wrought metal surface.

6. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a metal base containing at least adjacent the surface thereof a dispersion of finely divided stable metal oxide, coating the metal base surface with an intermediate layer of a powder mixture of metal oxide and metal powder, the amount of metal powder ranging from about 5 to 25% by volume of the mixture, thermally bonding said intermediate layer to the metal base surface, applying a layer of metal oxide on top of said intermediate layer and thermally bonding it to the intermediate layer, whereby an adherent oxide layer is formed on said metal surface.

7. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a metal base from the group titanium and zirconium having alloyed therewith up to about 1% by weight of a rare earth metal capable of forming by oxidation a finely divided disperse stable metal oxide having a negative free energy of formation at about 25 C. of at least 90,- 000 calories per gram atom of oxygen, subjecting said metal base to internal oxidation to produce a dispersion of finely divided oxide at least adjacent the surface thereof, coating the metal base surface with a layer of metal oxide and thermally bonding said metal oxide to said surface by forming a bond between said layer and the finely divided dispersed metal oxide at least adjacent the metal base surface.

8. A method for producing an adherent oxide coating upon a metal surface which comprises, providing a metal base from the group titanium and zirconium having alloyed therewith up to about 1% by Weight of cerium, subjecting said metal base to internal oxidation to form a dispersion of finely divided cerium oxide at least adjacent the surface thereof, coating the metal base surface with a layer of metal oxide and thermally bonding said metal oxide to said surface by forming a bond between said layer and the finely divided dispersed metal oxide at least adjacent the metal base surface.

9. An oxide-coated metal base comprising a metal oxide coating bonded to said metal base via a dispersion of finely divided metal oxide particles at least adjacent the metal base surface, said dispersed metal oxide particles having a negative free energy of formation at about 25 C. of at least 90,000 calories per gram atom of oxygen, said bonding being obtained via the intrusion .of said oxide layer into said metal base surface in connecting relationship with the finely divided oxide particles dispersed in the metal at least adjacent the surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,704,586 Beck et al. Mar. 5, 1929 2,340,884 Kinzie et al Feb. 8, 1944 2,492,682 Carpenter et al. Dec. 27, 1949 2,823,988 Grant et al. Feb. '18, 1958 2,883,283 Wainer Apr. 21, 1959 2,894,838 Gregory July 14, 1959 2,972,529 Alexander et al. Feb. 21, 1961 OTHER REFERENCES Journal of the Institute Metals, vol. 83, May 1955, pp. 417420 relied on.

C. R. Cupp: Progress in Metal Physics, vol. IV, 1953, 

1. A METHOD FOR PRODUCING AN ADHERENT OXIDE COATING UPON A METAL SURFACE WHICH COMPRISES, PROVIDING A METAL BASE CONTAINING AT LEAST ADJACENT THE SURFACE THEREOF A DISPERSION OF FINELY DIVIDED STABLE METAL OXIDE HAVING A NEGATIVE FREE ENERGY OF FORMATION AT ABOUT 25* C. OF AT LEAST 90.000 CALORIES PER GRAM ATOM OF OXYGEN, PRODUCING AN OXIDE COATING ON SAID METAL SURFACE, AND THERMALLY BONDING SAID COATING TO SSAID METAL SURFACE VIA BONDS FORMED BETWEEN SAID OXIDE COATING AND THE DISPERSED METAL OXIDE AT LEAST ADJACENT THE METAL BASE SURFACE. 